Directional couplers, which are electronic devices that can detect power being transmitted in a particular direction, are used in a wide variety of radio frequency (RF) circuits. For example, a directional coupler may be used in a radar system to detect a reflected wave by separating the indecent wave from the reflected wave, or may be used in a circuit that measures the impedance mismatch of transmission lines. Functionally, a directional coupler has a forward transmission path and a coupled transmission path. The forward transmission path generally has a low loss, while the coupled transmission path couples a fraction of the transmission power that is propagated in a particular direction. There are many different types of coupler architectures that include electromagnetic couples and magnetic couplers. Each of these coupler types may be implemented using different topologies and materials depending on the frequency of operation and the operational environment.
Generally, the directivity of a directional coupler is sensitive to the termination at the coupled and isolated ports. For example, if the impedance of the termination varies with frequency, the directivity of the directional coupler may also vary with frequency. The variation of impedance with frequency may be due to parasitic capacitances/inductances present at the termination ports. The impedance of the termination may also deviate from the expected value due to random variations or temperature variations. One way this issue has been conventionally addressed is by using a dual-directional coupler having two directional couplers connected in series.